**3.2 Macroelement content of digestate**

The other characteristics of digestate also are differed depending on the source materials and the digestion process. In Table 2 some major properties of different liquid digestates can be seen, but these are mean values which could be altered in the course of the digestion process. Therefore regular monitoring of digestate properties is needed in the case of its agricultural use.


Table 2. Characteristics of liquid digestates from different origin

Nitrogen (N) is a major plant nutrient and is the most common plant growth limiting factor of agricultural crops. The fertilizing effect of added N is decreased by the inadequate synchrony of crop N demand and soil N supply (Binder et al., 1996; Möller & Stinner, 2009). The advantage of digestate application is the possibility of reallocation of the nutrients within the crop rotation from autumn to spring, when crop nutrient demand arises (Möller

Digestate: A New Nutrient Source – Review 299

and 38.4% in the case of different ingestates, highest loading rates and hydraulic retention times while Marcato et al. (2008) found this value of 53%. If the organic loading rate of biogas plant is high and the hydraulic retention time is short, the digestate will contain a considerable amount of undigested OM, which is not economic and not results a good amendment material. However, the OM content of digestate is more recalcitrant and therefore the microbial degradation and soil oxygen consumption can be decreased by its

The adequacy of digestate as soil amendment is based on its modified OM content. Most OM is converted into biogas, while the biological stability of remaining OM was increased during AD with the increase of more recalcitrant molecules like lignin, cutin, humic acids, steroids, complex proteins. These aliphatic and aromatic molecules are possible humus precursors with high biological stability (Tambone et al., 2009). Pognani et al. (2009) found the increase of these

> Lignin (g kg-1 TS)

Table 3. Changes in macromolecules content on the course of AD *(Data from Pognani et al.,* 

Similarly, the rate of lignin-C, cellulose-C and hemicellulose-C are increased in the organic matter content after AD of cattle and pig dung (Kirchmann & Bernal, 1997). The increase of these macromelecules-C were 2.4-26.8 %, 14.2-13.9 % and 7.3 % in the manures, respectively. The hemicellulose-C content in the anaerobically treated pig dung was decreased by 23.8 %. However, the increase of non-decomposable carbon content of digestate is always smaller than that of composts (Gómez et al., 2007). On the other hand, improving the fertilizer effect of a digestate with its higher decomposable carbon content results in an increase in roots and crop residues which may have an important effect on the soil organic matter content.

Digestate is a very complex material therefore its using has effect on the wide range of physical, chemical and biological properties of the soil, depending on the soil types (Makádi et al., 2008). The recycled organic wastes are suitable for contribution to maintain the soil nutrient levels and soil fertility (Tambone et al., 2007). Among the organic amendments the ratio of liquid digestate in the agriculture is known to be around of 10%. It can be applied as

Digestate

Ingestate Hemicelluloses (g kg-1 TS)

> Digestate

Ingestate

127 35 49 280 35 42 50 68

143 36 72 243 27 54 71 79

Celluloses (g kg-1 TS)

> Digestate

Ingestate

macromolecules′ quantities in the course of AD as it can be seen in Table 3.

Total solid (TS) (g kg-1 ww)

> Digestate

Ingestate

**4. Effects of digestate on soil properties** 

application (Kirchmann & Bernal, 1997).

Type of ingestate

Energy crops, cow manure slurry and agro-industrial waste

Energy crops, cow manure slurry, agro-industrial waste and OFMSW

*2009)*

et al., 2008). The higher N content of a digestate comparing to the composts is the consequence of the N concentration effect because of carbon degradation to CO2 and CH4 and N preservation during AD (Tambone et al., 2009).

The NH4 content of the digestate is about 60-80% of its total N content, but Furukawa and Hasegawa (2006) reported 99% of NH4-N of the digestate originated from kitchen food wastes. Generally, the NH4-N concentration is increased by the protein-reach feedstock (Kryvoruchko et al., 2009) like diary by-products and slaughterhouse waste (Menardo et al., 2011). The conversion of organic N to NH4-N allows its immediate utilization by crops (Hobson and Wheatley*,* 1992). The higher amount of NH4-N and the higher pH predominate over the factors (lower viscosity, lower dry matter content) which could reduce the ammonia volatilization from the digestate (Möller & Stinner, 2009). The emission of ammonia could be decreased by different injection techniques which lower the air velocity above the digestate and because of the bound of gaseous ammonia to soil colloids and soil water (McDowell and Smith, 1958). The application depth has a significant effect on ammonia volatilization. Surface application of a liquid biofertiliser caused the loss of 20-35% of the applied total ammoniacal N while disc coulter injection into 5-7 cm depth reduced the ammoniacal loss to 2-3% (Nyord et al, 2008). This method should be used also in the case of digestate application to reduce ammonia volatilization.

Digestate has higher phosphorus (P) and potassium (K) concentration than that of composts (Tambone et al., 2010) therefore it is more suitable for supplement of these missing macronutrients in soils. Furthermore, Börjesson and Berglund (2007) assumed all phosphorus in the digestate to be in available forms, therefore digestate seems to be a useful material for supplement missing nutrients of soil, especially of the P and K. The average phosphorus-potassium ratio of digestates is about 1:3 which is excellent for grain and rape. Accumulation of P and K in soil could be avoided by the reduction of the applied digestate dose but in this case, for the supplement of nitrogen gap, the artificial fertilizer has to be used.
